The disclosure of Japanese Patent Application No. 2001-001594 filed on Jan. 9, 2001 and No. 2001-021759 filed on Jan. 30, 2001, each including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to an automatic transmission that shifts rotation of an input shaft at a selected one of a plurality of speed ratios and transfers the thus-shifted rotation to an output shaft by engaging and disengaging control clutches and control brakes connected to various elements of a speed-changing dual planetary gear unit connected to the input shaft.
2. Description of Related Art
U.S. Pat. No. 5,106,352 discloses an automatic transmission that includes: a speed-changing dual planetary gear unit having first and second sun gears, a long pinion that directly meshes with the first sun gear and that meshes with the second sun gear via an intermediate pinion, a carrier that supports the long pinion and the intermediate pinion, and a ring gear that meshes with the long pinion and that is connected to an output shaft; and a speed-reducing planetary gear having a ring gear connected to an input shaft, a sun gear fixed to a transmission case, and a carrier that supports a pinion meshed with the ring gear and the sun gear. This automatic transmission achieves six forward gear ratios and one reverse gear ratio by selectively transferring the rotation of the carrier of the speed-reducing planetary gear unit that has been reduced in speed so that the rotation speed of the carrier is less than the rotation speed of the input shaft to the second and first sun gears through the use of first and third control clutches, and by selectively transferring the rotation of the input shaft to the carrier of the speed-changing dual planetary gear unit through the use of a second control clutch, and by selectively restricting the rotations of the first sun gear and the carrier of the speed-changing dual planetary gear unit through the use of first and second control brakes.
This conventional automatic transmission has a small entire length, and is therefore suitable for a front-wheel drive vehicle with a transversely mounted engine. In recent years, however, there is a demand for an automatic transmission capable of achieving seven or more forward gear ratios that are appropriately spaced, in order to improve fuel economy and power transfer performance or to provide gear ratios that match a driver""s taste.
Accordingly, it is an object of the invention to provide a high-efficiency automatic transmission that achieves seven or more forward gear ratios so as to allow optimal extraction of engine performance in a high vehicle speed region and reduces changes in output torque during gear shifting by adding a gear speed including a locked-up speed at a high gear speed side so that adjacent gear ratios are closer to each other. This results in smooth gear to gear transitions and, thus, a smoother, more pleasurable ride.